Re: [PATCH v2] rust: types: add FOREIGN_ALIGN to ForeignOwnable

["Benno Lossin" <lossin@kernel.org>]

On Tue Jun 10, 2025 at 4:15 PM CEST, Alice Ryhl wrote:
> On Tue, Jun 10, 2025 at 4:10 PM Andreas Hindborg <a.hindborg@kernel.org> wrote:
>>
>> "Benno Lossin" <lossin@kernel.org> writes:
>>
>> > On Tue Jun 10, 2025 at 1:30 PM CEST, Andreas Hindborg wrote:
>> >> diff --git a/rust/kernel/types.rs b/rust/kernel/types.rs
>> >> index 22985b6f6982..0ccef6b5a20a 100644
>> >> --- a/rust/kernel/types.rs
>> >> +++ b/rust/kernel/types.rs
>> >> @@ -21,15 +21,11 @@
>> >>  ///
>> >>  /// # Safety
>> >>  ///
>> >> -/// Implementers must ensure that [`into_foreign`] returns a pointer which meets the alignment
>> >> -/// requirements of [`PointedTo`].
>> >> -///
>> >> -/// [`into_foreign`]: Self::into_foreign
>> >> -/// [`PointedTo`]: Self::PointedTo
>> >> +/// Implementers must ensure that [`Self::into_foreign`] returns pointers aligned to
>> >> +/// [`Self::FOREIGN_ALIGN`].
>> >>  pub unsafe trait ForeignOwnable: Sized {
>> >> -    /// Type used when the value is foreign-owned. In practical terms only defines the alignment of
>> >> -    /// the pointer.
>> >> -    type PointedTo;
>> >> +    /// The alignment of pointers returned by `into_foreign`.
>> >> +    const FOREIGN_ALIGN: usize;
>> >>
>> >>      /// Type used to immutably borrow a value that is currently foreign-owned.
>> >>      type Borrowed<'a>;
>> >> @@ -39,18 +35,20 @@ pub unsafe trait ForeignOwnable: Sized {
>> >>
>> >>      /// Converts a Rust-owned object to a foreign-owned one.
>> >>      ///
>> >> +    /// The foreign representation is a pointer to void. Aside from the guarantees listed below,
>> >
>> > I feel like this reads better:
>> >
>> > s/guarantees/ones/
>> >
>> >> +    /// there are no other guarantees for this pointer. For example, it might be invalid, dangling
>> >
>> > We should also mention that it could be null. (or is that assumption
>> > wrong?)
>>
>> It is probably not going to be null, but it is allowed to. I can add it.
>>
>> The list does not claim to be exhaustive, and a null pointer is just a
>> special case of an invalid pointer.
>
> We probably should not allow null pointers. If we do, then
> try_from_foreign does not make sense.

That's a good point. Then let's add that as a safety requirement for the
trait.

---
Cheers,
Benno